In infants and young children formalin-inactivated RSV vaccine induced a high level of binding (ELISA) antibodies to the RSV F glycoprotein but these antibodies were deficient in both neutralizing and fusion-inhibiting activity. Radioimmunoprecipitation of post-immunization sera revealed that antibodies to the internal viral M and NP antigens were also produced by a majority of the vaccinees tested. Thus, formalin- inactivated vaccine induced high levels of antibodies to the M and NP antigens as well as the non-functional antibodies to the viral surface glycoproteins may have acted in concert to produce a pulmonary Arthus reaction which contributed significantly to the potentiation of disease that occurred when vaccinees were infected with RSV. Immunization of infants and children who had not been infected with RSV previously led to a potentiated disease in 70 to 80% of these vaccinees, whereas immunized individuals who had been infected previously had a much lower frequency of disease potentiation. The IgG subclass antibody response to the F and G glycoproteins of RSV of infants and children undergoing first, second, and third infection with RSV revealed that the IgG1 response to the G glycoprotein was deficient and that the IgG2 response to the heavily glycosylated G glycoprotein was neither age dependent nor different from that to the F glycoprotein. The antigenic relatedness of the RSV subgroup A and subgroup B strains was analyzed using sera from infants who had a primary infection with RSV. The F glycoproteins of subgroup A and subgroup B strains were highly related, whereas the G glycoproteins were only 5% related. Relatedness between subgroups was 30% when analyzed by cross-neutralization.